Current function generator



Aug. 19, 1969 MASAKAZU SHOJI 7 3,462,617

CURRENT FUNCTION GENERATOR Filed Jan. 20, 1967 2 Sheets-Sheet 1 uvvewronM. SHOJ/ ATTO NEV Aug,- 1969- MASAKAZU SHOJI I 3,462,617

:CURRENT FUNCTION GENERATOR 2 Sheets-Sheet 2 Filed Jan. 20, 1967 FIG. 7

FIG. 6

United States Patent Office 3,462,617 Patented Aug. 19, 1969 U.S. Cl.307-460 7 Claims ABSTRACT OF THE DISCLOSURE Two-valley semiconductordevices exhibiting a traveling high field domain upon application of abias voltage have a current characteristic that varies with doping leveland/or cross-sectional area. Specific current waveforms are produced byvariations of these parameters along the length of the device.

BACKGROUND OF THE INVENTION This invention relates to current functiongenerators and, more particularly, to such devices utilizing the Gunneffect.

Current functions, that is, currents of specialized or particularwaveforms, are of utility in a number of applications such as, forexample, analog computers, logic circuits, test equipment, and ascurrent sources for display devices. In many applications, highfrequency or high speed is required of the function generator. Desiredwaveforms and speed may be obtained by relatively com-.

plex circuitry involving the use of numerous passive and active devicessuch as transistors and tunnel diodes. Obviously, a reduction in circuitcomplexity and the number of elements used without sacrificing eitherspeed or fidelity of waveform are highly desirable.

SUMMARY OF THE INVENTION The present invention is a high speed currentfunction generator that utilizes the Gunn effect to achieve high speedof operation and extreme simplicity. It is based upon the discovery thata two-valley semiconductor, i.e., Gunn effect, device, such as aproperly doped gallium arsenide crystal, produces an output current thatis substantially an exact replica of the geometric shape of the crystal.

In a Gunn effect device, upon application of suflicient voltage, anarrow high-field domain is nucleated at the cathode and travels towardthe anode with a substantially constant velocity and current density.During the transit of this domain from cathode to anode, theinstantaneous current flowing through the device is given by the productof the current density and the cross-sectional area of the device at thelocation of the domains. As a consequence, the output current waveformdepend upon the crosssectional area of the device and is substantially areplica thereof. As will be discussed more fully hereinafter, thecurrent Waveform can also be made to follow variations in dopingconcentration, which variations produce changes in the current densityduring passage of the domain through the sample. Because of thedependence of current waveform upon the geometry or dopingconcentration, it is possible to produce a variety of Waveforms thatcannot be produced by the switching actions which typify many of theprior art devices.

The present invention will be more readily understood from the followingdetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of oneillustrative embodiment of the invention;

FIG. 2 depicts the current waveform produced in the arrangement of FIG.1;

FIG. 3 is a diagram of another illustrative embodiment of the invention;

FIG. 4 depicts the current waveform of the arrangement of FIGURE 3;

FIGS. 5 and 6, respectively, depict a particular crystal shape and theresulting waveform; and

FIGS. 7 and 8, respectively, depict still another crystal shape and theresulting waveform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is shown anembodiment of the invention for generating a triangular currentwaveform. The arrangement of FIG. 1 comprises a crystal 11 of galliumarsenide or other suitable Gunn effect type material. In the case ofgallium arsenide a suitable dopant such as oxygen may be used to providesufficient carriers for achieving Gunn oscillations. Crystal 11 has athickness s, a length L, and a height b which varies between b and b asshown. At one end (x=o) of crystal 11 is a first contact 12 of suitablematerial such as, for example, indium,.and another such contact 13 is atthe other end (x=L) of crystal 11. Connected in series between contacts12 and 13 is a suitable voltage source 14 which may take any of a numberof suitable forms and an output or load resistor 16 at the ends of whichare output contacts 17 and 18. Load resistor 16 is intended to representany of a number of possible broadband loads or outputs which havesufficient bandwidth capacity to pass the current waveform producedwithout degradation.

In operation, upon application of a voltage from source 14 in excess ofthe Gunn effect threshold, a high field domain is nucleated adjacentcontact 12, i.e., the cathode. As is known, where crystal 11 isuniformly doped, this domain travels toward contact 13, i.e., the anode,with a substantially constant velocity v and substantially constantcurrent density I as long as the applied voltage is sufficient toproduce a domain sustaining field E The current density I is given by J87love With the thickness s held constant, the instantaneous currentvaries directly with the height b for a uniformly doped crystal. Thusthe current waveform is 'a replica of the shape of crystal 11. FIG. 2depicts the current waveford produced by the arrangement of FIG. 1. Forthe waveshape of FIG. 2, it is necessary that the bias voltage fromsource 14 be sufliciently large to sustain the traveling domainthroughout the length of the sample. If the voltage is insufficient tosustain the domain at b the domain collapses and a sawtooth waveformresults instead of that shown in FIG. 2. Where the voltage is sufficientto sustain the domain throughout the length L, a current spike occurswhen the domain arrives at the anode, i.e., contact 13. This spike isshown in dashed lines in FIG. 2. This current spike may be eliminatedwhen necessary.

For a waveform as shown in FIG. 2, for a fundamental frequency of, forexample, megacycles, the circuitry associated with the device of FIG. 1should be sufficiently broadband to support the tenth harmonic, i.e.,1,000 megacycles. This is in contrast to other Gunn effect devices wherethe associated circuitry is narrowband, passing primarily the singlefrequency of interest. In general, the load circuitry to which theoutput of a function generator is applied is sufficiently broadband tosustain the current waveform without degradation.

In FIG. 3 there is shown a variation of the arrangement of FIG. 1 whichproduces the waveform shown in FIG. 4. Crystal 21, as can be seen,differs from crystal 11 of FIG. 1 only in the variations in height. Inall other respects the arrangements are the same, hence the samereference numerals have been used. The bandwidth requirements for thefunction generator of FIG. 3 and waveform of FIG. 4 differ somewhat fromthose of FIGS. 1 and 2. For each different waveform the bandwidthrequirements must be determined on the basis of passing the highestharmonic necessary to maintain the particular waveform. Thisdetermination is, in every case, within the capabilities of thoseskilled in the art.

FIGS. 5 through 8 show other examples of crystal shapes 31 and 41 andtheir associated waveforms. It can be seen that many possible waveformsare possible through application of the principles of the presentinvention.

As was pointed out before, and as can be seen from Equations 1 and 2,changes in doping which produce changes in J can be used to accomplishthe same ends. The particular crystal shapes shown in FIGS. 1, 3, 5, and7 would then represent the doping profile rather than changes in the bdimension. Techniques for producing variations in doping levels areknown to workers in the art.

In one embodiment of the invention, crystal samples were made fromn-type oxygen doped GaAs having an electron concentration of 4 to 7 10/cm. The crystals were cut into 40 x 40 mil squares from to mils thickand ground to the desired shape. The shaped samples were cleaned,etched, and alloyed with pure indium contacts. In operation the crystalswere biased by a 50 nanosecond pulse train of a mercury relay pulsehaving a repetition frequency of 120 Hz. The current waveforms through a4-ohm monitoring resistor were observed on a sampling oscilloscope.

From the foregoing, it is readily apparent that a wide variety ofwaveforms may be generated utilizing the principles of the presentinvention. All that is necessary is a single active element, i.e., aGunn efiect crystal, cut to the desired shape, and associated circuitryhaving sufficient bandwidth to pass the desired waveform. This is incontradistinction to high speed function generators of the prior art,which usually consists of complex combinations of both active andpassive elements.

Numerous arrangement utilizing the principles of the present inventionmay occur to workers in the art without departure from the spirit andscope of the present invention.

What is claimed is:

1. Apparatus comprising:

a semiconductor wafer of the type that is capable of forming andpropagating traveling electric field domains in response to theapplication of a sufficient bias voltage;

cathode and anode contacts connected to the wafer;

means for applying sufiicient voltage to the contacts to cause travelingdomains to propagate through the wafer, the velocity of propagationbeing a substantially constant velocity v said apparatus beingcharacterized by means for generating current waveforms each of whichwaveforms has an amplitude I that varies significantly throughout asubstantial portion of its period as a predetermined function of time t;

said waveform generating means comprising substantial variations in theproduct n A with distance throughout a substantial portion of the regionthrough which the domain propagates, wherein n is the dopingconcentration, A is the cross-sectional area of the device and v t isapproximately the distance in the region of domain propagation from thepoint where the domain is formed;

I(t) being a linear function of the product n A;

said apparatus being connected to a circuit of suflicient bandwidth tomaintain the waveform of the current through said device.

2. A two-valley semiconductor device as claimed in claim 1 wherein thedoping level is substantially uniform along the length of the device andthe cross-sectional area of said device varies along the length thereof.

3. A two-valley semiconductor device as claimed in claim 2 wherein onedimension only of said device varies along the length thereof.

4. A two-valley semiconductor device as claimed in claim 1 wherein thecross-sectional area of said device is substantially constant along thelength thereof and the doping level varies.

5. The apparatus of claim 1 wherein the current waveforms that aregenerated are sinusoidal and the variation in the product n A issinusoidal.

6. The apparatus of claim 1 wherein the current waveforms are triangularand the product n A increases linearly to an intermediate point in theregion through which the domain propagates and decreases linearly fromthereon.

7. The apparatus of claim 1 wherein the current waveforms are sawtoothedin shape and the product n A increases linearly throughout the -regionthrough which the domain propagates.

References Cited UNITED STATES PATENTS 8/1956 Shockley. 1/1968 Gunn.

OTHER REFERENCES JOHN W. HUCKERT, Primary Examiner JERRY D. CRAIG,Assistant Examiner U.S. Cl. X.R.

